The main characters are Phaedria, a young Athenian man of good family, and Thaïs, a courtesan he loves. Chaerea, Phaedria’s younger brother, and Parmeno, Phaedria’s slave, also play important roles. Laches, the brothers’ father, figures significantly although he is seldom onstage. Thraso, a wealthy, arrogant soldier, and his “parasite” (a sycophant associated with a higher-status person) Gnatho also complicate the plot. Pamphila, a young woman who grew up with Thais, is central to many plot twists, especially regarding her identity as a free citizen or a slave. Chremes is her brother.

The diverse statuses of Athenians and people of nearby city-states are important features of The Eunuch. Well-to-do families had slaves, who were usually brought to Athens from different areas, and slave status often effectively blocked marriage with a citizen or wealthier person. Phaedria’s love for Thaïs and his dedication to helping her restore Pamphila to her rightful place and her brother’s care are two qualities that mark his character’s good nature. In addition, he must make the difficult decision to punish his brother for raping Pamphila, showing that such behavior was not considered acceptable toward a slave. While Thaïs is of lower status because she is a courtesan, she is also the head of her own establishment; while she manipulates Thraso, it is toward a positive end, to rescue the woman who is as close as a sister. Her innate goodness is redemptive and can make her a suitable partner for a higher-born man.

The individualization of the characters, in general, is not pronounced. They are largely variants of the stock characters from Greek theater that were adapted by the Roman playwrights. Gnatho and Thraso, the fawning flatterer and the boastful soldier to whom he attaches himself, are one typical pair. Similarly, Parmeno embodies the clever slave who helps his less-intelligent or less-wordly master navigate complicated situations.